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PIVOT_ROOT(2) Linux Programmer's Manual PIVOT_ROOT(2)

pivot_root - change the root mount

int pivot_root(const char *new_root, const char *put_old);

Note: There is no glibc wrapper for this system call; see NOTES.

pivot_root() changes the root mount in the mount namespace of the calling process. More precisely, it moves the root mount to the directory put_old and makes new_root the new root mount. The calling process must have the CAP_SYS_ADMIN capability in the user namespace that owns the caller's mount namespace.

pivot_root() changes the root directory and the current working directory of each process or thread in the same mount namespace to new_root if they point to the old root directory. (See also NOTES.) On the other hand, pivot_root() does not change the caller's current working directory (unless it is on the old root directory), and thus it should be followed by a chdir("/") call.

The following restrictions apply:

  • new_root and put_old must be directories.
  • new_root and put_old must not be on the same mount as the current root.
  • put_old must be at or underneath new_root; that is, adding some nonnegative number of "/.." prefixes to the pathname pointed to by put_old must yield the same directory as new_root.
  • new_root must be a path to a mount point, but can't be "/". A path that is not already a mount point can be converted into one by bind mounting the path onto itself.
  • The propagation type of the parent mount of new_root and the parent mount of the current root directory must not be MS_SHARED; similarly, if put_old is an existing mount point, its propagation type must not be MS_SHARED. These restrictions ensure that pivot_root() never propagates any changes to another mount namespace.
  • The current root directory must be a mount point.

On success, zero is returned. On error, -1 is returned, and errno is set appropriately.

pivot_root() may fail with any of the same errors as stat(2). Additionally, it may fail with the following errors:
EBUSY
new_root or put_old is on the current root mount. (This error covers the pathological case where new_root is "/".)
EINVAL
new_root is not a mount point.
EINVAL
put_old is not at or underneath new_root.
EINVAL
The current root directory is not a mount point (because of an earlier chroot(2)).
EINVAL
The current root is on the rootfs (initial ramfs) mount; see NOTES.
EINVAL
Either the mount point at new_root, or the parent mount of that mount point, has propagation type MS_SHARED.
EINVAL
put_old is a mount point and has the propagation type MS_SHARED.
ENOTDIR
new_root or put_old is not a directory.
EPERM
The calling process does not have the CAP_SYS_ADMIN capability.

pivot_root() was introduced in Linux 2.3.41.

pivot_root() is Linux-specific and hence is not portable.

Glibc does not provide a wrapper for this system call; call it using syscall(2).

A command-line interface for this system call is provided by pivot_root(8).

pivot_root() allows the caller to switch to a new root filesystem while at the same time placing the old root mount at a location under new_root from where it can subsequently be unmounted. (The fact that it moves all processes that have a root directory or current working directory on the old root directory to the new root frees the old root directory of users, allowing the old root mount to be unmounted more easily.)

One use of pivot_root() is during system startup, when the system mounts a temporary root filesystem (e.g., an initrd(4)), then mounts the real root filesystem, and eventually turns the latter into the root directory of all relevant processes and threads. A modern use is to set up a root filesystem during the creation of a container.

The fact that pivot_root() modifies process root and current working directories in the manner noted in DESCRIPTION is necessary in order to prevent kernel threads from keeping the old root mount busy with their root and current working directories, even if they never access the filesystem in any way.

The rootfs (initial ramfs) cannot be pivot_root()ed. The recommended method of changing the root filesystem in this case is to delete everything in rootfs, overmount rootfs with the new root, attach stdin/stdout/stderr to the new /dev/console, and exec the new init(1). Helper programs for this process exist; see switch_root(8).

new_root and put_old may be the same directory. In particular, the following sequence allows a pivot-root operation without needing to create and remove a temporary directory:


chdir(new_root);
pivot_root(".", ".");
umount2(".", MNT_DETACH);


This sequence succeeds because the pivot_root() call stacks the old root mount point on top of the new root mount point at /. At that point, the calling process's root directory and current working directory refer to the new root mount point (new_root). During the subsequent umount() call, resolution of "." starts with new_root and then moves up the list of mounts stacked at /, with the result that old root mount point is unmounted.

For many years, this manual page carried the following text:

pivot_root() may or may not change the current root and the current working directory of any processes or threads which use the old root directory. The caller of pivot_root() must ensure that processes with root or current working directory at the old root operate correctly in either case. An easy way to ensure this is to change their root and current working directory to new_root before invoking pivot_root().

This text, written before the system call implementation was even finalized in the kernel, was probably intended to warn users at that time that the implementation might change before final release. However, the behavior stated in DESCRIPTION has remained consistent since this system call was first implemented and will not change now.

The program below demonstrates the use of pivot_root() inside a mount namespace that is created using clone(2). After pivoting to the root directory named in the program's first command-line argument, the child created by clone(2) then executes the program named in the remaining command-line arguments.

We demonstrate the program by creating a directory that will serve as the new root filesystem and placing a copy of the (statically linked) busybox(1) executable in that directory.


$ mkdir /tmp/rootfs
$ ls -id /tmp/rootfs    # Show inode number of new root directory
319459 /tmp/rootfs
$ cp $(which busybox) /tmp/rootfs
$ PS1='bbsh$ ' sudo ./pivot_root_demo /tmp/rootfs /busybox sh
bbsh$ PATH=/
bbsh$ busybox ln busybox ln
bbsh$ ln busybox echo
bbsh$ ln busybox ls
bbsh$ ls
busybox  echo     ln       ls
bbsh$ ls -id /          # Compare with inode number above
319459 /
bbsh$ echo 'hello world'
hello world


/* pivot_root_demo.c */
#define _GNU_SOURCE
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/syscall.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <limits.h>
#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                        } while (0)
static int
pivot_root(const char *new_root, const char *put_old)
{
    return syscall(SYS_pivot_root, new_root, put_old);
}
#define STACK_SIZE (1024 * 1024)
static int              /* Startup function for cloned child */
child(void *arg)
{
    char **args = arg;
    char *new_root = args[0];
    const char *put_old = "/oldrootfs";
    char path[PATH_MAX];
    /* Ensure that 'new_root' and its parent mount don't have
       shared propagation (which would cause pivot_root() to
       return an error), and prevent propagation of mount
       events to the initial mount namespace */
    if (mount(NULL, "/", NULL, MS_REC | MS_PRIVATE, NULL) == 1)
        errExit("mount-MS_PRIVATE");
    /* Ensure that 'new_root' is a mount point */
    if (mount(new_root, new_root, NULL, MS_BIND, NULL) == -1)
        errExit("mount-MS_BIND");
    /* Create directory to which old root will be pivoted */
    snprintf(path, sizeof(path), "%s/%s", new_root, put_old);
    if (mkdir(path, 0777) == -1)
        errExit("mkdir");
    /* And pivot the root filesystem */
    if (pivot_root(new_root, path) == -1)
        errExit("pivot_root");
    /* Switch the current working directory to "/" */
    if (chdir("/") == -1)
        errExit("chdir");
    /* Unmount old root and remove mount point */
    if (umount2(put_old, MNT_DETACH) == -1)
        perror("umount2");
    if (rmdir(put_old) == -1)
        perror("rmdir");
    /* Execute the command specified in argv[1]... */
    execv(args[1], &args[1]);
    errExit("execv");
}
int
main(int argc, char *argv[])
{
    /* Create a child process in a new mount namespace */
    char *stack = malloc(STACK_SIZE);
    if (stack == NULL)
        errExit("malloc");
    if (clone(child, stack + STACK_SIZE,
                CLONE_NEWNS | SIGCHLD, &argv[1]) == -1)
        errExit("clone");
    /* Parent falls through to here; wait for child */
    if (wait(NULL) == -1)
        errExit("wait");
    exit(EXIT_SUCCESS);
}

chdir(2), chroot(2), mount(2), stat(2), initrd(4), mount_namespaces(7), pivot_root(8), switch_root(8)

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2019-08-02 Linux